Oxygen or a mixture of air and oxygen is often supplied to a patient in a hospital or nursing home. The oxygen can be supplied on a demand basis, or it can be used for forced or induced ventilation if the patient experiences difficulty in voluntary breathing. In any event, it is usually desired to monitor the volume and rate of flow of oxygen or other gas to the patient. Furthermore, it often is desired to measure the patient's capability of exhaling. The prior venturi devices for this purpose have been limited in operation and accuracy by accumulation of water condensed from the patient's exhaled breath. In such prior art venturi devices, enough water could accumulate in fifteen to thirty minutes to affect readings adversely.